The present invention relates to a method and apparatus for reducing mechanical stresses on the shaft of turbosets and on the generator shaft coupled thereto upon the occurrence of three-pole mains short circuits near the generators and upon the elimination of such short circuits.
Examination of the influence of short circuits in the vicinity of a power plant on the mechanical stresses on the shafts of turbosets has shown that three-pole mains short circuits near the generator and their elimination with simultaneous return of the full line voltage may lead to very high mechanical stresses on the shafts. These stresses excede by a multiple those of a two-pole short circuit -- a case of stress for which the shafts are designed (ETZ-A, vol. 95 (1974), pages 508-514). The reasons for this stress are the following circumstances:
When a short circuit occurs near the generator, that means within a range of about 1 km from the power station, the load is removed from the generator and assuming that the short circuit does not create an arc, an inductive short circuit current with low losses flows. On the other hand, collapse of the voltage removes the synchronizing moment with the result that the generator is accelerated by the initially still unthrottled turbine, causing the load angle to increase under these conditions the machine would become asynchronous if the short circuit were not eliminated very quickly, i.e., within about 200 ms. The mechanical stresses on the shaft of the turboset occurring during the short circuit are relatively low. If the short circuit is now switched off by one of the circuit breakers on the high voltage side, then the phase shifted line voltage of the network 8 reappears at the generator and a synchronizing moment is developed. Depending on the mechanical starting conditions at the moment of return of the voltage, this moment leads to transient mechanical oscillations at relatively high amplitude.
In order to attempt to avoid these large amplitude oscillations the above-mentioned publication proposes that the short circuit not be simultaneously switched in all three poles but to initially switch off only one pole. Thus the three-pole short circuit would be converted to a two-pole short circuit and thereafter, with an offset of a defined period of time, the other two poles would be switched off. This will considerably reduce the mechanical stresses. However, the mechanical stresses still lie above the stresses for which the shaft was designed.